1. Field of the Invention
The present invention relates to an ejector for ejecting fluid and a fuel cell system employing the ejector.
2. Description of Related Art
Development of fuel cells such as PEFCs (Polymer Electrolyte Fuel Cells), generating electricity by use of hydrogen (fuel gas, reactant gas) supplied to an anode and air containing oxygen (oxidizer gas, reactant gas) supplied to a cathode, is accelerating in recent years.
A fuel cell system generally has a stacked cell structure, in which some tens to hundreds of fuel cells are stacked in order to increase sufficiently an output voltage. Each cell is formed by sandwiching an MEA (Membrane Electrode Assembly) between a pair of separators. The MEA includes two electrodes (anode, cathode) and a solid polymer electrolyte film sandwiched between the electrodes. With hydrogen gas (as the fuel) supplied to the anode and air containing oxygen (as the oxidizer) supplied to the cathode, the fuel cell generates electricity by electro-chemical reaction occurring on the electrodes.
In a fuel cell system employing such a fuel cell, hydrogen gas is supplied from a hydrogen tank to the fuel cell while anode-off gas is discharged from the fuel cell. Since the amount of hydrogen contained in the hydrogen gas supplied from the hydrogen tank is larger than that necessary for the electricity generation, the anode-off gas discharged from the fuel cell contains unreacted hydrogen. For the effective use of hydrogen, the anode-off gas is circulated by making use of a circulating apparatus, mixed with the hydrogen gas from the hydrogen tank, and supplied (returned) to the fuel cell.
The circulating apparatus is in many cases implemented by an ejector, which circulates the anode-off gas utilizing pressure energy without a need of any external power source.
A fuel cell system employing the ejector generally needs a regulator for changing the pressure of hydrogen supplied to the fuel cell depending on the amount of generated electric power that is required (i.e., the amount of electricity or electric power that should be generated), a device for switching/adjusting the opening diameter of a nozzle of the ejector to achieve suitable circulating performance in response to the amount of the required electric power to be generated.
For the regulator and the device for switching/adjusting the opening diameter (opening area) of the nozzle of the ejector, a variety of techniques have been devised, as disclosed in Japanese Laid-open Patent Publications No. 2002-227799, No. 2002-056868, No. 2004-095528 and No. 2005-183357, for example.